LNG is a natural gas that has been cooled to a cryogenic condition to condense methane, the natural gas main component. A temperature of approximately −161 C is required to produce and keep natural gas in a liquid state at standard atmospheric pressure. Liquefaction reduces the volume by approximately 600 times thus making it more economical to transport over great distances versus traditional pipelines. At present, LNG is primarily transported across continents thus making it available throughout the world. LNG is also produced in small scale liquefaction plants to supply peak saving demands, as well as to make available natural gas to regions that need it but where it is not economical or technically feasible to build pipelines.
There are differences in liquefaction selection processes for large versus small LNG plants. For large plants, the main criteria is minimization of capital cost whereas the minimization of energy consumption is left as a second objective. These two objectives can also go together; thus an optimization of the efficiency of the plant may involve a reduction in the investment of the equipment. On the other hand, a higher efficiency can result in an increase in LNG production, so the efficiency factor has a significant impact on the plant economics. In small to medium LNG plants, it is not the efficiency, but other factors such as simplicity, modularization, ease of maintenance, operation and installation that have an higher criteria when selecting a liquefaction technology. The direct consequence of these different selection criteria is that liquefaction technologies for small to medium scale applications are not the same as the ones that are used in large LNG plants.
The two main groups of liquefaction technologies are the mixed refrigerant technologies and expansion based technologies. The mixed refrigerant technologies are “condensing type” processes, where the refrigerant used for the liquefaction makes use of its latent heat of vaporization to cool the natural gas. The expansion based technologies are processes where the refrigerant is always in gas phase and only makes use of its sensible heat to cool the natural gas.
The following mixed refrigerant technologies are the most representative processes in the industry: PRICO (Poly Refrigerated Integrated Cycle Operation) is licensed by Black and Veatch and it consists of one cycle of mixed refrigerant (a mixture of methane, ethane, propane, butane, nitrogen and sometimes isopentane), the advantages claimed by the licensor are operating flexibility, modular design and reduced refrigerant inventory. The AP-M (Air Products) is licensed by APCI, is a single mixed refrigerant that is vaporized at two different levels of pressure. The dual pressure cycle is more efficient than the single pressure cycle, resulting in smaller heat exchangers and compressor. The LiMuM (Linde Multistage Mixed Refrigerant) is licensed by Linde and consists of a spiral wound heat exchanger and one 3-stage single mixed refrigeration loop for the pre-cooling, liquefaction and sub-cooling of the natural gas. This process allows for high capacity throughput. PCMR (Pre-cooled Mixed Refrigerant) is licensed by Kryopak and consists of a pre-cooling stage (ammonia or propane cycle) followed by a single mixed refrigerant cycle, where the mixed refrigerant is a mixture of nitrogen, methane, ethane, propane and butanes, this process is used primarily in small plants. OSMR (Optimized Single Mixed Refrigerant) is licensed by LNG Limited, the process is a single mixed refrigerant process complemented with a standard package ammonia absorption process. The utilization of an ammonia process improves the efficiency of the process and an increase in LNG output compared to traditional single mixed refrigerant processes. In all of the above mixed refrigerant technologies, the main differences between them are the composition of the mixed refrigerant (although the refrigerants are the same i.e.; nitrogen, methane, ethane, etc.), the metallurgy of the heat exchangers, the orientation of the equipment and the operations set points. In all the mixed refrigerants processes, the objective of innovation is to increase efficiency, reducing capital and operating costs.
The expansion based technologies have various processes based on the use of nitrogen as a refrigerant to liquefy natural gas, the N2 expansion cycle. Some of these processes use a single cycle, others use a dual expansion cycle and in other cases a pre-cooling cycle is added to improve efficiency. Several licensors i.e., APCI, Hamworthy, BHP Petroleum Pty, Mustang Engineering and Kanfa Oregon offer the N2 expansion cycles processes, and they differ by proprietary process arrangement. In all these processes, the cooling is provided by an external refrigeration plant using nitrogen expanders. The Niche LNG process is licensed by CB&I Lummus, consists of two cycles: one cycle uses methane as a refrigerant and the other uses nitrogen. The methane provides cooling at moderate and warm levels while the nitrogen cycle provides refrigeration at the lowest temperature level. The OCX process is licensed by Mustang Engineering and is based on the use of the inlet gas as a refrigerant in an open refrigerant cycle with turbo-expanders, there are variations such as OCX-R which adds a closed loop propane refrigerant to the OCX process and OCX-Angle which incorporates LPG recovery.
As demonstrated, presently there are many variations and processes to liquefy LNG. All of the processes operate based on the expansion of low boiling fluids be it through expanders or JT valves, be it closed or open cycle, the difference between them is in the process efficiencies which result in lower capital and operating costs per unit of LNG produced.
What is required is an alternative method to liquefy gas, such as LNG.